APPLICANT'S ABSTRACT: As part of our ongoing efforts of studying the effects of alcohol (EtOH) on the brain, it is the goal of this proposal to investigate the mechanism of alcohol-induced tolerance in humans using state-of-the-art in-vivo magnetic resonance (MR) methodology. Three major hypotheses will be tested: 1) alcohol-tolerance is associated with reduced compartmentation of EtOH in brain membranes after EtOH consumption; 2) phospholipid membranes of tolerant subjects, compared to those of nontolerant controls, have increased rigidity which does not resolve after 6 - 9 months of abstinence; and 3) increased membrane rigidity and reduced compartmentation of EtOH in brain membranes are associated phenomena. These hypotheses are based on MR spectroscopy (MRS) work performed by others and by ourselves showing: a relatively weak proton (1H) MRS signal of free EtOH in the brain (compared to temporaily concordant blood alcohol concentrations), stronger EtOH signal in the brain of chronic, heavily drinking (HD) subjects compared to light/non-drinking (LD) controls at similar blood alcohol concentrations, and a reduced phosphorus (31P) MRS signal from membrane phospholipids in the brain of HD subjects. It is thought that these findings may be due to increase rigidity of phospholipid membranes in HD subjects, that the reduced visibility of EtOH and the reduced phospholipid signal intensities in HD subjects compared to LD controls are related, and that both observations are expressions of tolerance to alcohol. This proposal is aimed at replicating and extending these earlier findings using multi-volume proton (1H) and 31P MR spectroscopic imaging methods to: 1) measure the concentration of free EtOH in the brains of HD and LD subjects after EtOH ingestion; 2) determine whether there is a second (MR-invisible) pool of EtOH possibly associated with brain membranes; 3) estimate the relative amount of this pool in LD and HD subjects; 4) determine in the brains of HD and LD subjects and abstinent alcoholics (AA) both the T2 relaxation times (which reflect membrane rigidity) and the concentrations of phospholipid signals, and 5) to determine whether an association exists between the increased EtOH signal after acute EtOH administration and the reduced phospholipid MRS signal in HD subjects. The studies will give a mechanistic explanation for higher EtOH MR-visibility in alcohol-tolerant subjects, and will lead to a greater understanding of the cellular mechanisms underlying alcohol tolerance. They may provide a tool for investigating the association between structural membrane changes and hyperexcitability during alcohol withdrawal and may form the basis for an objective, quantitative invivo measure of alcohol tolerance; this may be valuable for predicting an alcoholic's vulnerability to relapse. Finally, the proposed studies may provide means for determining whether there is an inherited membrane rigidity reflecting innate tolerance to alcohol in subject groups at genetic high risk of alcoholism. REVIEW A